Johnson_AHS02 - Influence of Wake Models on Calculated...

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1 Influence of Wake Models on Calculated Tiltrotor Aerodynamics Wayne Johnson Army/NASA Rotorcraft Division NASA Ames Research Center Moffett Field, California Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 1/4-scale V- 22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of measured and calculated performance and airloads for helicopter mode operation, as well as calculated induced and profile power and wake geometry. The focus of this paper is on the further development of wake models for tiltrotors in helicopter mode operation. Three tiltrotor wake models are considered, characterized as the rolled-up, multiple-trailer, and multiple-trailer with consolidation models. By using a free wake geometry calculation method that combines the multiple-trailer wake model with a simulation of the tip vortex formation process (consolidation), good correlation of the calculations with TRAM measurements is obtained for both performance and for airloads. Notation . a speed of sound A rotor disk area, p R 2 c n blade section normal force coefficient, N/( 1 / 2 r U 2 c) c ref blade reference chord C P rotor power coefficient, P/ ( W R) 3 A = Q/ ( W R) 2 RA C T rotor thrust coefficient, T/ ( W R) 2 A (shaft axes) C X rotor propulsive force coefficient, X/ ( W R) 2 A (wind axes, positive forward) G strength of trailed vorticity M 2 c n blade section normal force coefficient times Mach number squared, N/( 1 / 2 a 2 c) M tip blade tip Mach number, W R/a ____________ . Presented at the American Helicopter Society Aerodynamics, Acoustics, and Test and Evaluation Technical Specialists Meeting, San Francisco, CA, January 23-25, 2002. Copyright ' 2002 by the American Helicopter Society International, Inc. All rights reserved. N number of blades N blade section normal force r blade radial station (0 to R) r c vortex core radius r C centroid of vorticity r G moment (radius of gyration) of vorticity R blade radius P rotor power, P = W Q q dynamic pressure, 1 / 2 V 2 Q rotor torque T rotor thrust (shaft axes) X rotor propulsive force (wind axes, positive forward) V wind tunnel speed a , s rotor shaft angle (positive aft, zero for helicopter mode) m advance ratio, V/ W R air density s rotor solidity, Nc ref / R ( = 0.105 for TRAM) y blade azimuth angle (zero azimuth is downstream) W rotor rotational speed
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2 Introduction The tiltrotor aircraft configuration has the potential to revolutionize air transportation by providing an economical combination of vertical take-off and landing capability with efficient, high-speed cruise flight. To achieve this potential it is necessary to have validated analytical tools that will support future tiltrotor aircraft development. These analytical tools must calculate tiltrotor aeromechanical behavior, including performance, structural loads, vibration, and
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Johnson_AHS02 - Influence of Wake Models on Calculated...

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